Lateral flow devices have been described and utilized for detecting analyte(s) present in a test sample. An exemplary conventional lateral flow device is illustrated in FIG. 1C. The lateral flow test device 10 typically includes a reagent zone and a test zone 13. The reagent zone includes a sample receiving zone 11 and a label zone 12. The test zone 13 can contain a test result zone 132 and a control test result zone 133 located downstream of the test result zone 132. Typically, the sample receiving zone contains a porous sample receiving pad 111 and the label zone contains a conjugate pad 121, both of which constitute the reagent zone where all necessary assay reagents are contained therein. The test zone is typically in form of a test strip 131 with a test result zone 132 and a control test result zone 133 located downstream. A calorimetric readout appearing in the test result zone typically indicates the presence of the analyte being tested. The lateral flow device typically contains an absorbent pad 141, which is in fluid communication with the elements 11, 121, and 131 along the direction indicated by the arrow of FIG. 1C. Conventional lateral device are made of nitrocellulose strip or nylon which are immobilized with binding agents that bind the analyte being tested. See, for example, the lateral flow test devices described in U.S. Pat. Nos. 4,857,453; 5,073,484; 5,119,831; 5,185,127; 5,275,785; 5,416,000; 5,504,013; 5,602,040; 5,622,871; 654,162; 5,656,503; 5,686,315; 5,766,961; 5,770,460; 5,916,815; 5,976,895; 6,248,598; 6,140,136; 6,187,269; 6,187,598; 6,228,660; 6,235,241; 6,306,642; 6,352,862; 6,372,515; 6,379,620 and 6,403,383, each incorporated herein by reference in their entirety.
Either competitive or non-competitive assays can be performed with the conventional lateral flow test devices for detecting the presence of an analyte in a liquid sample. However, such devices and methods are not particularly amenable to adjusting detection thresholds, which allow for a positive or negative signal to be produced at a predetermined analyte concentration. In addition, these devices and methods are not particularly useful for measuring analytes at low concentrations with high degree of accuracy. When an analyte is present at very low concentration in a liquid sample, a buffer solution is required to extract the analyte from the sample, which can affect the result and create inconvenience and raise safety concerns to the person who is handling the assay. Therefore, there exists a need for testing devices that require minimum manual operation while ensuring accurate and reliable test results.